The present invention relates to an intake air flow rate measuring apparatus for use in an internal combustion engine and, more particularly to an intake air flow rate measuring apparatus for use in an internal combustion engine for an automobile. The intake air flow rate measuring apparatus measures an intake air flow rate necessary for an execution of a fuel supply amount which is supplied into an automotive internal combustion engine.
The present invention relates to an intake air flow rate measuring apparatus for use in an internal combustion engine such as a hot wire type air flow meter or a hot film type air flow meter for measuring an air flow rate by utilizing a cooling action of an air flow on a heat generating temperature dependent resistor disposed in an air flow passage. The hot wire type air flow meter or the hot film type air flow meter according to the present invention is suitably applicable to the measurement of an intake air flow rate of an automotive internal combustion engine.
As a conventional intake air flow rate measuring apparatus for use in an internal combustion engine, there has been known a technique that an intake air flow rate measuring apparatus such as a hot wire type air flow meter has a drive processing circuit for heating a heat generating temperature dependent resistor and for processing various electric signals, for example, as disclosed in U.S. Pat. No. 4,400,974.
The conventional hot wire type air flow meter adopts a constant temperature type control circuit utilizing an operational amplifier as the drive processing circuit. The constant temperature type control circuit in the hot wire type air flow meter carries out a constant temperature control for the heat generating temperature dependent resistor.
The constant temperature type control circuit constituting the drive processing circuit in the conventional hot wire type air flow meter has a following circuit structure. Namely, voltage is applied to a series resistor, which comprises a heat generating temperature dependent resistor and a current detecting resistor, and further the voltage, which is applied to the above stated series resistor, is adjusted by making as an input therefor with a partial voltage output by the series resistor.
Besides, in an automotive internal combustion engine, the high frequency noises occur very often by the ignition spark and the discharge accompanying the electric supply to a distributor, and the radiation from the mobile ham transceiver is more serious. Concretely, in the conventional hot wire type air flow meter, since both a flow velocity measuring resistor constituting a heat generating temperature dependent resistor and a temperature compensating resistor constituting a temperature dependent resistor are exposed in an air flow passage, there is a phenomenon that high frequency noises intrude through an exposed portion, in which both resistors are exposed in the air flow passage, in the hot wire type air flow meter.
Therefore, in case that the high frequency noises intrude through the drive processing circuit in the hot wire type air flow meter, since the operational amplifier constituting the drive processing circuit is made an error operation, and the voltage applied to the series resistor is made also an error operation, accordingly an error operation occurs in the drive processing circuit in the conventional hot wire type air flow meter.
As a result, there is short of the reliability on the partial voltage output by the series resistor in the drive processing circuit, which indicates the intake air flow rate, in the conventional hot wire type air flow meter.